On the topic of writing 'half sectors' due to a power drop: the author of http://lwn.net/Articles/351521/ has done quite a lot of tests on various hard drives, and generally found that they usually don't do this, though some instances have done. He has a useful program that can test any drive for this behaviour, though it's mostly intended to test for out of order writes due to caching - I believe only some drives lie about this.
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